1. Field of the Invention
The present invention relates to the field of geomechanical modeling. More specifically, the present invention relates to the analysis of earth stresses associated with hydrocarbon recovery processes.
2. Background of the Invention
A subterranean hydrocarbon-bearing reservoir is confined by a state of in-situ tectonic stress. When producing hydrocarbons from the reservoir, the stress state of the reservoir may change. When the state of stress within and above the hydrocarbon-bearing reservoir exceeds the mechanical limits of wells completed in the production area, the completion assemblies forming the wells may be damaged.
A concern may also exist with respect to the effect that fluid removal from the reservoir may have at the earth surface. In this respect, in situ hydrocarbons provide pore pressure which acts against the overburden and which supports the rock strata above the formation. The removal of hydrocarbons, particularly when the rock matrix in the subsurface reservoir is weak, causes a reduction in pore pressure. When this occurs, the weight of the overburden is increasingly supported by the rock matrix, causing a compaction of the subsurface formation in response to the increased stress. This, in turn, can cause a subsidence of the earth at the surface.
The inverse can also occur in connection with fluid injection. Injection operations may be conducted as part of enhanced oil recovery, such as the injection of steam or brine into a producing formation. Injection operations may also occur simply in connection with a water disposal program. The injection of fluids into the subsurface formation will cause an increase in pore pressure within the targeted formation. This, in turn, can create stresses in the formation that may affect wellbore casings. Further, increased pore pressure may cause heave at the surface of the earth.
It may be desirable for the operator to predict the likelihood or extent of earth movement as a result of subsidence or heave. In some instances, earth movement is sought to be controlled in order to avoid environmental or hydrogeological impact. In this respect, changing the contour and relief of the earth surface can change runoff patterns, affect vegetation patterns, and impact watersheds.
It may also be desirable to predict local changes in the in-situ state of stress and the impact of such changes on well integrity over the life of reservoir production. Compaction particularly has the potential of damaging producers or injectors formed in a production area. In this respect, downward earth movement can create damaging hoop and compressional stresses on wellbore casings, cement jobs, and downhole equipment. When substantial deformation occurs, a well may lose its capability to permit remediation and may even block fluid passage. At this stage, the casing needs to be repaired or the well plugged and abandoned, and a new well created.
To anticipate changes in geomechanical stress, it has been proposed to use an integrated geomechanical and reservoir analysis. Newer and more sophisticated measurement techniques have demonstrated that variations in reservoir deliverability are related to interactions between changing fluid pressures, rock stresses and flow parameters. For instance, Young's modulus and Poisson's ratio are related to porosity.
It is desirable to model changes in geomechanical stress through finite element analysis. Finite element analysis involves the representation of individual, finite elements of a geological system in a mathematical model, and the solution of the model in the presence of a predetermined set of boundary conditions. Changes to the system are predicted as fluid pressures change.
In finite element modeling, the region that is to be analyzed is broken up into sub-regions called elements. The process of dividing a production area under study into sub-regions may be referred to as “discretization” or “mesh generation.” A mesh is a collection of elements that fill a space, with the elements being representative of a system which resides in that space. In finite element modeling, the region that is to be analyzed is represented by functions defined over each element. This generates a number of local functions that are less complicated than those which would be required to represent the entire region.
Finite element models have been used for analyzing production-induced earth stress changes associated with hydrocarbon recovery processes. For example, U.S. Pat. No. 6,766,255 describes a method of determining subsidence in a producing reservoir. However, it is desirable to have an improved geomechanical modeling method that automatically builds a three-dimensional map-based model from subsurface data, and then converts the map-based model into a finite-element-based model. A need also exists for a geomechanical model that corrects nonconformities in the earth layers, thereby accounting for pinchouts and erosive zones. A need further exists for an improved method for modeling a reservoir that takes into account changes in geomechanical stress over a period in the life of a reservoir. In addition, a need exists for a systematic method for assessing injector or producer well reliability by computer simulation.